Variable gain electron multiplier



Feb. 25, 1947. M. CAWEIN VARIABLE GAIN ELECTRON MULTIPLIER v Filed Jan. 14, 1944 2 Sheets-Sheet 1 INVENTOR' MADISON CAWEI N jl/ /l Feb. 25, 1947. M. CAWEIN 2,416,376

VARIABLE GAIN ELECTRON MULTIPLIER F iled Jan. 14,. 1944 2 Sheets-Sheet 2 TRANSMITTER INVENTOR MADISON CAWEIN 1 is a diagrammatic representationof Patented Feb. 25, 194'? VARIABLE GAIN ELECTRON MULTIPLIER Madison CaweinpFort Wayne, Ind., 'assignor to Farnsworth Television and Radio Corporation, a corporation of Delaware Application January 14, 1944, Serial No. 518,223

19 Claims. 1 This invention relates to electron multipliers and particularly to automatic gain control ape paratus therefor.

A conventionally employed multistage electron v multiplier is capable of amplifying electron streams of varying concentrations in a substantially linear manner; that is, the voltages developed in the output circuit of such a multiplier are substantially proportional to the electron concentrations in the primary electron streams from which they are-developed. The voltages are related to the corresponding electron stream concentrations by the multiplication factor of the multiplier. The overall multiplication factor of the multiplieris a function of the number of multiplying stages.

It is well known in the art that an electron multiplier of this character, when employed with apparatus for producing electron, streams varying in concentration throughout a substantial range of values, will develop voltages varying between maximum and minimum limits, which frequently are beyond the rangethat may-be usefully employed. It, therefore; is desirable toprovide apparatus to operate a multistage electron multiplier in such a, manner that the range of output circuit voltages is prevented from exceeding'a predetermined value.

' It,' therefore, nan object of'thepresent invention to provide control apparatus responsivetothe electron concentrations in. an electron multiplier for automatically controlling the multiplication. ratio thereof 'in a manner to preventthe voltage derived fronithe multiplier'from exceeding a predetermined maximum value.

Inaccordance with the invention, there is provided an electron multiplier having a plurality of secondary electron emissive electrodes and a collector'electrode connected to an output circuit.

Associated with one or more of the electrodes is I switching apparatus which is rendered operative in response to predetermined voltages developed iri'the output circuit to vary the number of operative electrodes whereby to correspondingly vary the multiplicationv factor of the device.

' "For abetter understanding of the invention,

together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, and its scope will be pointed out'in the appended claims.

In the accompanying drawings: 1

'a dissectortube'embodiment of the'inventicn; j s

Fig. 2 is a schematic'wiring'dlagramshowing and. the anode l3. f a

s disposed on they outside of the tube envelope ll in a manner to surround 2 the electrical interconnections of the pertinent components of a dissector tube drawn to an enlarged scale; and,

Fig. 3 is a fragmentary schematic diagram showing a modified form of the invention.

Havi'ngreference now to Fig. 1 of the drawings, there is illustrated a sectional view of a disformed a primary scanning aperture l5 which; is

disposed substantially centrally with respect to the cathode l2 and faces the cathode. Ani'accelerating anode is provided in the form of an interior wall coating 16 between the cathode. l2

A focusing coil H is substantially completely the space ,withinfthe tube between the cathode and the anode. A battery l8 or other suitable source of electrical energy is connected to the focusing coil for the energization thereof to establish the desired focusing field within the tube envelope whereby an electron image is formed substantially in the planeof the anode I3. The tube also is provided with horizontal and vertical scanning coils l9 and 2|, respectively. Each of these coils is energized by respective sources 22 and '23. of appropriate saw-toothed .wave form voltages.

In conjunction. with the dissector tu'be there also is provided an optical system represented by a lens 24, whereby an optical image of a subject 25 is focused into the plane of the photoelectric cathode l2.

Referring now to Fig. 2 of the drawings, a more detailed descriptionof the electrical connections to the photoelectric cathode,'the electron multiplier andthe switchingapparatus willbe given. Themultiplier which is housed within the anode shield l3 comprises a plurality of box-like electrodes suchas 26, 21, 28, 29. and 3|. In the case of a so-called eleven stagemultiplier, as illustrated herein, the device includes eleven of such boxlike electrodes. The inner surfaces of the electrodes are treated with material such as caesium, for the purpose of producing a relatively large number of secondary electrons by the bom- I bardment of the surfaces with arelatively small number of primary electrons. Adjacent'the out- A turestz 3 let of the eleventh stage electrode 3! is located an electron collecting electrode of suitable form such as a plate 32. The first stage electrode26 is provided With a secondary scanning aperture 33 in alignment with the primary scanning aperture 15 formed in the anode shield 53. The secondary aperture ordinarily is smaller than the primary aperture, being of substantially the same size as the desired elemental scanningareaf Suitable accelerating potentials are'impressed upon the multiplier electrodes and the photoelec tric cathode by appropriate connections to taps ona voltage divider which, asillustrated, comprises the series connection oi resistors 34 to, 45, inclusive. A source of relatively high potential unidirectional energy such as a battery 46 is con-f nected to the terminals of the voltage divider. The negative terminaljof the battery 46-is con-' nected to the photoelectric cathode I 2. Thejunc tion point between resistors 34 and 35 is connectedftothe shielding anode l3 and. also by means of an internal connection suchas represented by a conductor 41, to the first multiplier electrode 26. Thus, the anode shield and the first stage electrode are operated at the same positive potential with respect to the anode 7| 2. Similarly,

the electrodes comprising multiplier stages 2 to 8 inclusive, the latterof which comprises the elec- .trod'e 21, are connected directly to other rrespective taps on thevoltagef divider in] a manner ;to

succeeding multiplier electrodes.

'trodes 28,29}! or 32.

Referring now to the operation of the appara- .tus of'Figs. l and 2 embodying the present invention, assume that initially'the maximum intensity of the lightrefiected from the subject is relatively small. In this case the maximum impress increasingly higher positive potentials on The ninth; tenth and eleventh stage electrodes 2B, 29 ands! respectively; are connected to the rema ining taps onthe voltage divider through switching circuits controlled by three relays 48, ,49'and 5i. Itis contemplated to be within the scope of the invention thatthe'switchingcircuits be controlled by' equivalent electronic relays in a .suitablemanner. The-continuity 'of these con nections is determined by the operative crinoperative conditions 'of the relays. Normally; with alljrelaysinj theirunoperated positions; the con,-

xilQO'tibll between the last three multiplierelectrodes are substantially the same as those for any of the other, electrodes. The ninthjstage elecjtlOde .28 .isconnected through the, winding of relay .51 to the junction point between voltage ,divider resistors 42 and 43. I The tenth stage electrode: 29

isconnectedthrough thebackconta'ctassociated witharmature 'lizlofrelay 5| and the winding of relay 49 to the junction point between thedivider resistors 43 and Q4. 1 The eleventhstageelectrode 3! is connected through the back contact associated with armature 53 of relay 49 and the wind illg of relay ditto the junction point between the divider resistors 44 and 35. 'The'fcolle'ctor electrode 32 is connected through thebr'eak contact I associated with armature 354, of relay stand-an output circuit'resistor 55-to the grounded positive terminal of the battery 38. The collector electrode also is connected through the serially arranged break contacts associatedwith armatures and a, coupling condenser 59lto thecontrol grid v :of an amplifier tubefil. .A leak resistortz also isconnected between the control grid of thetube and ground. .The output circuit; ofthe, amplifier tube Bl, which 'includiesthe" anode and the grounded cathode, is connected to thei'input circuit terminalsof a transmitter 61%. The output terminals of the transmitter are connected to, a

suitable transmission channel which mayjinclude ara iator'aa; I Y V Stand:fid respectively, of relays I concentration of any of the electron streams from the photoelectric cathode 12 Which are admitted to the multiplier is not in excess of that required to develop an output voltage 'of a predetermined maximum tolerable value. In this case all ofthe switching relays 48, 49 and 5! are intheirfunoperated positions as shown. The collector. electrode 32 thus .is connected through the break contact associated Witharma'ture M to the normal output circuit resistor 55. In this case the fiow of current through this resistor resulting from the collection of electrons by the electrode t 32 develops signal voltages inthe resistor which are impressed by the circuit including the break contacts associated With armatures' 55, 51, and '58 and the coupling condenser 59 upon tl' e input 'circuit'of'the amplifiertube at}; at the same t m by reason of the described electron accelerating potential connections to the ninth, tenth and eleventh rnu'ltiplier stages 28, 29 and 3! jrespec-.

tiv'ely, there is produced a current flow through the operating windings of the'relays 487,149 and 5:. 'The magnitudes ofthe respective fcurrents is .a function of the electron, currentfiow'to the respective electrodes and this current flow is dependent upon the electron concentrations atjthe respective electrodes. Thus, the maximum cu'rrent traverses'the Windingof relay 48 while curofrelays Hand 54. N

It now' the reflected illumination from the subject 25 increases so as to effect an. increase in jrents of lesser magnitudes traverse the windings the electron concentration of the stream representative of one ofthe brighter elemental areas of thesubject, there will be effected an increase in the electron collection; by the electrode .32. Consequently, the greatestvoltage developed in thejoutput resistor 55 Will'exceed the predeter-- mined maximum tolerable gvalue. In this case there also will be proportional increasesin, the magnitudes of the currents traversin'gthe windings of relays 48, 49' and 5|. I Under these conditions the relays are adjusted so thatthe current then traversing the winding of relay' 428- will be suificient to actuate this relay thereby moving itsassociated armatures 54 andifi into engagement with their respective make/contacts.

The circuit from the outputrsis'tor' 55 to the collector electrode 3 2 is opened at the break contact associated with armaturefi l, At, the same time. this electrode. is connected, by the make. contact associated with this armature to any electron repelling voltage such'as the negative terminal of the battery 46. The normal eleventh stage multiplier electrode 3! .remains jconnected through the winding of. relay 48 to the voltage divider. This electrode also. is now connected .through'th'e make contact associated witharmac re-ave ture 56 and the break contacts associated with armatures 51 and 58 to the coupling condenser 59.

It is seen, therefore, that the normal collector electrode 32 is disconnected from the normal output resistor 55 and at the same time is connected to a negative potential point for the purpose of rendering it inoperative for the attraction thereto of electrons from the preceding stage. At the same time the normal eleventh stage electrode 3|, by reason of its 'normal connection through the impedance represented by the winding of relay 48 is rendered operative as a collector electrode to develop signal voltages across the impedance of the winding of relay 48. Also, by reason of the switching of this electrode to the input circuit of the amplifier tube 6| the signal voltages developed in the manner described are impressed upon the amplifier.

It is noted that the normal voltage of the electrode 3|, when acting as the eleventh stage of the multiplier, is 180 out of phase with the voltage developed at the electrode 32 when acting as the electron collector of the multiplier. When the described switching operation has been performed the phase of the voltage developed at the electrode 3| is the same as the normal voltage developed at the electrode 32. This result is obtained by rendering ineilective in the manner described the electrode 32 to attract electrons from the electrode 3|. The latter electrode then functions a an electron collector and consequently develops voltages of the same phase as the normal collector voltages.

It is seen, therefore, that after the described switching operation has been performed one-electron multiplying stage is eliminated from the multiplier. Consequently, the voltage developed across the winding of relay 48 will be of considerably les magnitude than the voltage developed across the resistor 55 just prior to the performance of the switching operation. By this means then the maximum signal voltage which is impressed upon the input circuit of the amplifier tube 6| is prevented from exceeding the predetermined maximum tolerable value.

If now, as a result of a still further increase in the maximum reflected illumination from the subject 25 the voltage developed at the acting electron collector electrode 3| reache the predetermined maximum tolerable value for impression upon the amplifier tube 6 the electron current flow through the winding of relay 49 to the normal tenth stage multiplier electrode 29 will be increased sufiiciently to eifect the actuation of the relay armatures 53 and 51. The electrode 3| is disconnected at the break contact associated with armature 53 from its normal tap on the voltage divider and instead is connected by the make contact associated with this armature to the negative terminal of the battery 46. In this manner the electrode 3| is rendered ineffective for the attraction theretov of electron from the preceding electrode 29. At the same time the electrode 3| is disconnected at the break contact associated with armature 51 from the amplifier coupling condenser 59.v The electrode 29 is maintained in connection with its normal tap on the voltage divider through the winding of relay 49.

. prising the winding .of relay 4'9. These voltages are impressed, by means of the described connection to the coupling condenser 59, upon the input circuit of the amplifier tube 6|. Again, by reason of rendering electrode 3| ineffective to attract electrons from the electrode 29, the electrons are collected by the latter electrode and these signal voltages are in phase with the corresponding voltages previously impressed'upon the amplifier by either of the electrodes 3| or 32 when the latter were serving as collectors.

If the illumination reflected from the subject 25 again increases suiliciently to raise the signal voltage developed across the winding of relay 49 to the. maximum tolerable value, the electron current flow through the winding of relay 5| to the normal ninth stage multiplier electrode28 will be increased sufliciently to operate this re lay, causing the actuation of its armatures 52 and 58.

The electrode 29 thereby is disconnected at the break contact associated with armature 52 from its normal tap on the voltage divider and instead is connected through the make contact associated with this armature to the negative terminal of battery 46.

contact associated with armature 58 from the amplifier coupling condenser 59. At the same time the electrode 28 is maintained in connection with its normal point on the voltage divider through the winding of relay 5|, connected through the make contact associated with armature 58 to the coupling condenser 59. In this manner the signal voltages developed in the impedance comprising the winding of relay 5|, with the associated multiplier electrode 28 acting as an electron collector, are impressed upon the input circuit of the amplifier tube 6|.

It will be noted that, as each of the switching relays 49 and 5| is operated, the preceding relays 48 and 49 respectively, are released. For example, the circuit through the winding ofrelay '4 48 is opened at the break contact associated with armature 53 when the relay 48 is operated. similarly, the continuity of the circuit to the winding of relay 49 is broken at the break contact asso-f However,

the electrode 3| prevents the passage of elec-' trons beyond the preceding electrode 29. Thus,

there is no possibility of electrons reaching the collector electrode 32 to develop a voltage in the resistor 55. In like manner the reconnection of any of the multiplier electrodes to their normal tape on the voltage divider, after having been rendered inoperative with respect to the preced ing multiplier electrode, will not adversely affect the functioning of the apparatus as described.

If it is desired to switch the output circuit of a multistage electron multiplier to still earlier stage electrodes, additional relays connected similarly to those shown and described may be em- ;ployed. Such an arrangement is contemplated to The electrode 29 thus is rendered inoperative for the attraction of elec-T trons thereto from the preceding electrode 28." Also, the electrode 29 is disconnected at the break,

This electrode also is.

ia'g s a co e 'w and th a safies ee a dliq ls sa nj n be of o y ectedfelectron as a result ation, thev,cur-.

i k thss ih eti lq e t. i rent flow li lhithawin p fl e el fl wil s insufii cient Ltomaintain this relayg'operatedi In fro contact associated with armatureBZ to the multi:

plir lectrode 29, thereby effecting the operation inputfc zircuit of the amplifier tube 6| In a similar m'anner, relay 4 8 vis'treoperated in .the' event that? theillumination isstill further decreased;

Re r ren emw is made to Fig. 3 which shows a niodification of the'invention. In this case only the higher multiplier stages and their .asso-? ciated connections are illustrated, Apparatus which is the equivalent of corresponding apparatus shown in Fig. 2 is represented by similar characters of reference. Theninth, tenth and eleventh multiplier stages 28, 29fand 3| respectively ;a1'" ,e connected ,to appropriate taps on the voltage divider through the' resp'ective resistors 65, 66f;and 61. These .multiplier electrodes also:

are connected to the make contacts of the switch ing relays 68, 69 and IL. 'The collector electrode 3Zii's connected through an output resistor 55 to,

the grounded pos'itive,trminal'of the battery 46. This electrode also is connected to the control grid of a phase shifting vacuum tube '12 which also h sh cathode whichgis connected to ground. Spa eurrent for the tube is provided by a source ofjf'uriidirectional energy s'uchas afbattery 13;

having its negative terminal con nected to ground an tshositive terminaliconnected through the wi dingsof relays.88,69 and 7!, arranged in series o ths ano of'the tube. The output cirf cuit f the phase shifting tube (2, is derived from th node which also is' connected through the break contacts associated respectively with armaturesfjliand 16 f relays'68 '69 and llrespective 1y, to acouplingcondenser 71. This con densersis connected to the control grid of an amplifier tube 78 having its cathodeconnected to ground} Aisuitable biasing and leak circuit for the grid of vthe resistor ,8 i l The output circuit of the amplifiertube lil is;

derived from the anode and cathode thereof and it h p e i u r scuba m dific t of: the invention.

Reter ringnow to the operation of the fmodifir;

cation ofthe intention shewn inj'ig, 3, assume that, the average. su-bi ect, illumination intensity 1 is such toeiiect the development of output voltages. 1 in ii ew i qri ifiwh donut e ceed: i maa nitudei the predetermined maximum" tolerable.-

value when the entire multiplier structure is in operation. In this case, all, of the switching reth case the armatures 52 and 58! are disengagedf theirre'spective make contacts and-rev. eng. ged with their respective break contacts. myths, manner a circuitithroughthe v winding: of relay 49 is reestablishedfihrough the break 8. V acts assqsiated w th; arma'mre .3 6,v and; 14 espesti-velra lectrensi q le teibrrthsielectrodez a h urrents-flow; in; the u put. ci cu pfa the= phas r erersine. tube 2 is ,a ,functiollzOf; the numa. ber of electrons collected by the multiplier; $8190.11 trodetfiz This current, also traversesthetwind- 2,0 ings of; relays. 968,16 iand, llg, all of which, have= marginal operation. characteristics; The ;relay 68am:- xample, is adjusted to op rate at a ;rela,-- tim smallcurrent; The relay, 69 ,is adjustedso, that ,-itw will ;not, operate at :thelrelativelylsmalljr 5., operating V-currentnrequiredtlfor the relay 681011123:

willlgresnondhto ,a somewhatgreater current. Similarly,= therelay .l l. is, adjusted to \remain.unoperated; when energized by v the .minimum cure; rents required to operate vrelays 68 v and 69 1311111;

39 willgrespond to relatively large currents: V In the, case, where thevrvoltagegdeveloped in the-output; resiston 55 is not, in 1 vexcess of that required to produce, at the output circuit .Ofgthe; amplifier tube J8, :a voltage greaterlthan ithetpredetermined 5;; maximum ,tolerable value none of the switching;

at the break.contaot'.associatedhwithriarmature= 141013 .the operatedrelay; At the same time the eleventh 4 .mu-ltiplierdstagea electrode r3111 iis :COhe-z nected through the make contact associated with;-v armature Matcthe input circuitof .theamplifier ,tube.18;.. v It';iS;-n0ted-:tha-t in this formof the invention: thetmul-tiplier' electrodeswhich iszswitc'hed away:- from theoutput. circuit-is not renderedinopera-- tivetwith zrespect Ito. the other. multiplier electub 78 prbvided y source of r trodeswInv this case'the collector electrode 32 u'riid' ectioial energy such as a battery 79 and a maintained c-inzrconnection with thenormal cir cuit resistor so. that ilwhas impressed thereon a higher positive potentialtthan the eleventh stageelectrode-$.13 Consequently, the electron flow s, 4 I W .Y .t Leon-through the multiplier' to its normal output ciris connected to the input circu t of the transm1tcuit :is not mate'riallyz afiected" bythe described: switching operation.v With respectto :the:-de-'- velopment of the desired signal voltage the'only difierenceiis that, instead ofutilizing the-voltage developed in the lnor-mal output resistor 55, --'the-- voltages developedtin the resistor 5 1 connected I to'rthe eleventh :stag'e tele'ctrode 3 are employed; It-zis byreasonof. the use of these difierent types of sresistors that'the use of the-phase-sh-ifting '70 ltubeJZEisZrequiredin the-one case but not in the-- other. v

In v the functioning of a multistage I -electroni multiplierithere is a net lossof'elec'tron's ateach of :the secondaryr' missive electrodes; The elee 575 -vtrodes'ithus: tend to increase m positive polarity the amplifier 18.

derived from any of the other secondary emissive with an increase in the electron concentrations supplied to the multiplier. On the other hand there is a net gain of electrons by the collector electrode 32. Consequently, the potential of this electrode tends to becomeincreasingly negative with an increase of the electrode concentrations. Thus, it is apparentthat the voltages developed at anyof the multiplier electrodes are 180 out of phase with respect to the voltages developed at the collector electrode. Therefore, when the output circuit of the multiplier is switched from the collector electrode to any of the secondary emissive electrodes, it is necessary to effect a phase shift in the output circuit.- In the present instance this is accomplished by including the phase shifting tube 1T2 between the normal output circuit derived from the collector electrode and When the output circuit is electrodes, the phase shifting tube 12 is not included between the electrode and the amplifier v If now the illumination intensity increases still furthenthe relay 69 will be operated to connect at the make contact associated with armature 15 the tenth stage multiplier electrode 29 to the input circuit of the amplifier tube 18'. At the same time the eleventh stage multiplier electrode 3] is disconnected at the break contact associated with this armature from the amplifier.

. Similarly, an additional increase of the subject illumination will produce a current flow through the windings of the switching relays of suflicient magnitude to effect the operation of relay H, The electrode 29 thereby is disconnected at the break contact associated with armature 11 from the input circuit of the amplifier tube 18 and the ninth stage multiplier electrode is connected in place thereof by the make contactassociated with this armature.

As in the previously described embodiment of the invention,-additional relays, as desired, may he -connected to still'other multiplier electrodes in a manner similar to those illustrated.

A change of the subject illumination in the opposite sense will effect the successive releases of the switching relays in a sequence opposite to-that in which they are operated.

While there has been described what, at present, are considered .the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and therefore, it is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention. a

What is claimed is:

1. Electron multiplying apparatus comprising,-

a plurality of electrodes, a utilization circuit, and means responsive to signalcurrent derived from one of said electrodes for switching said utilization circuit from one to another of said electrodes,-

' whereby to prevent the development in said circuit of voltages exceeding apredetermin'ed maxi-- mum value.

2. Electron multiplying apparatus comprising, aplurality of electrodes, a utilization circuit nor- .mally connected to one of-said electrodes, and. switching means responsivev to,curren ts derived ijrom one of said electrodes to disconnect said utilization circuit from said normally connectedelectrode and to connect said:utilization circuit to another of said electrodes, whereby toprevent; the development in said utilization circuit: cf

tron collecting electrode, a utilization circuit normally connected to said collecting electrode, and switching means responsive to current derived from one of said electrodes to disconnect said collecting electrode from said utilization circuit and to connect in place thereof said secondary emissive electrode, whereby to prevent the development in said utilization circuit of voltages exceeding a predetermined maximum value.

4. Electron multiplying apparatus comprising, a plurality of secondary electron emissive elec-' trodes, an electron collecting electrode, a utilization circuit normally connected to'said collecting electrode, and switching means responsive to currents derived from one of said electrodes to disconnect said collecting electrode from said utilization circuit and to connect in place thereof any one of said secondary emissive electrodes, whereby to prevent the development in said utilization circuit of voltages exceeding a predetermined maximum value.

5. Electron multiplying apparatus comprising, a secondary electron emissive electrode, an electron collecting electrode, a utilization circuit, a relay-having contacts connected to said utilization circuit and to said collecting and secondary emissive electrodes, and an operating winding for said relay responsive to current derived from one of said electrodes for actuating the contacts of said relays to disconnect said collecting electrode from said utilization circuit and to connect in place thereof said secondary emissive electrode, whereby .to prevent the development in said utilization circuit of voltages exceeding a predetermined maximum value.

6. Electron multiplying apparatus comprising, a plurality of secondary electron emissive electrodes, an electron collecting electrode, an amplifier having input and output circuits, a plurality of relays having contacts connected to the input circuit of said amplifier and to said collecting and secondary emissive electrodes, and operating windings for said relays responsive to currents derived from said electrodes for actuating'the contacts of said relays to disconnect said collecting electrode from the input circuit of said amplifier and to connect in place thereof one of said secondary emissive electrodes, whereby to prevent the development in the output circuit of said amplifier of voltages exceeding a predeter' mined maximum value.

7. Electron multiplying apparatus comprising,

a plurality of secondary electron emissive electrodes, an electron collecting electrode, an am-- plifier having input and output circuits, a plurality of relays having contacts connected to the input circuit of said amplifier and to said collecting and higher stage secondary emissive elec trodes, and operating windings for said relays connectedfo'r response to currents derived from said electrodes for actuating the contacts of said v relays in response to derived currents of a predetermined magnitude to disconnect said collecting electrode from the input circuit of said amplifier and to connect in place thereof anyasecondaryelectron-emissive electrode, an el'ec-.

a secondary electron emissive electrode, an electron collecting electrode, respective circuits connected to said electrodes for the development of voltages of different magnitudes and opposite phase, a utilization circuit, means responsive to voltage changes in said collecting electrode circuit to switch said utilization circuit from one of said electrode voltage developing circuits to another, and means for reversing the phase of the voltage developed in said collecting electrode circult.

16. Electron multiplying apparatus comprising, a secondary electron emissive electrode, an electron collecting electrode, impedance devices connected to said electrodes, a utilization circuit, a relay having circuit switching contacts, a phase reversing device having input and output circults, the input circuit of said phase reversing device being connected to said collecting electrode and the output circuit thereof being connected through the contacts of said relays to said utilization circuit, connections from said secondary emissive electrode to contacts of said relay, and a winding for said relay responsive to a current flow in excess of a predetermined magnitude in the output circuit of said phase reversing device for effecting the operation of said contacts to disconnect the output circuit of said phase phase reversing device being connected to said collecting eletrode and the output circuit thereof being connected to the windings of said switching relays, the output circuit of said phase reversing device also being connected through the contacts of said relays to said utilization circuit, connections from said secondary emissive electrodes to contacts of said relays, and means responsive to a current flow in excess of a predetermined magnitude in the output circuit of said phase reversing device for effecting operations of said relays to disconnect the output circuit of said phase reversing device from said utilization circuit and to connect in place thereof one of said secondary emissive electrodes, whereby to prevent the development in said utilization circuit of voltages exceeding a predetermined maximum value.

18. Electron multiplying apparatus comprising, a plurality of secondary electron emissive electrodes, an electron collecting electrode, means including resistance elements for impressing electron accelerating potentials upon said secondary emissive electrodes, a utilization circuit, a plurality of marginal relays each having an operating winding and circuit switching contacts, a vacuum tube having input and output circuits, the input circuit of said vacuum tube being connected to said collecting electrode and the output circuit thereof being connected to the windings of said switching relays, the output circuit of said vacuum tube also being connected through the contacts of said relays to said utili zation circuit, connections from said secondary emissive electrodes to respective contacts of said relays, and means responsive to current flows in excess of predetermined magnitudes in the output circuit of said vacuum tube for effecting the successive operations of said relays to disconnect the output circuit of said Vacuum tube from said utilization circuit and to connect in place thereof one of said secondary emissive electrodes, whereby to prevent the development in said utilization circuit of voltages exceeding a predetermined maximum value.

19. Electron multiplying apparatus comprising,

a plurality of secondary electron emissive electrodes, an electron collecting electrode, a voltage divider having a plurality of taps for impressing electron accelerating potentials upon said secondary emissive electrodes, resistance elements connecting respective taps on said voltage divider to said collecting electrode and to a plurality of higher stage secondary emissive electrodes, an amplifier having input and output circuits, a plurality of marginal relays each having an operating winding and circuit switching contacts, a phase reversing tube having input and output circuits, the input circuit of said phase reversing tube being connected to said collecting electrode and the output circuit thereof being connected to a series arrangement of the windings of said switching'relays, the output circuit of said phase reversing tube also being connected through the contacts of said relays to the input circuit of said amplifier, connections from said higher stage multiplier electrodes to respective normally open contacts of said relays, and means responsive to current flows in excess of predetermined ma itudes in the output circuit of said phase reversing tube for effecting the successive operations of said relays to disconnect the output circuit of said phase reversing tube from the input circuit of said amplifier and to connect in place thereof any one of said higher stage multiplier electrodes, whereby to prevent the development in the output circuit of said amplifier of voltages exceeding a predetermined maximum value.

MADISON CAWEIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED, STATES PA'IENTS 

